1. Field of the Invention
The present invention relates to a radiation noise suppression circuit in a differential transmission line which is one of high-speed signal transmission systems.
2. Related Art of the Invention
Recently, in flat-panel displays represented by liquid crystal TV sets and plasma TV sets, acceleration of signal speed at the time of transferring image information has progressed as it has become high definition from VGA (video Graphics Array) to XGA (eXtended Graphics Array). Then, a differential transmission system in low amplitude has been used increasingly as a high-speed digital data transmission system.
This transmission system is a transmission system which sends signals with opposite phases mutually through one pair of balanced cables or two wiring patterns formed on a printed wiring board. Its features are low noise, low voltage amplitude, high-speed data transmission, and the like, and its introduction has been progressing in a field of displays as a high-speed transmission method.
FIG. 6 is a structural diagram showing an example of a differential transmission line using an LVDS (Low Voltage Differential Signaling) system which is one of general differential transmission systems.
In FIG. 6, a plus line signal wiring 4a and a minus line signal wiring 4b whose differential impedance is 100 Ω connect a differential driver 1 and a differential receiver 2, and are terminated by a 100-Ω terminating resistor 3 near an input terminal of the differential receiver 2. Since signals with the same amplitude but opposite phases are applied to the two signal wiring 4a and 4b respectively, magnetic fields generated from the respective signal wiring 4a and 4b cancel each other, and hence, radiation noise hardly generates.
However, in actual differential transmission, since there is a skew, i.e., difference between rise time and fall time in signals outputted from the differential driver 1, a common mode current flowing with making the two signal wiring 4a and 4b outward circuits, and making a signal ground 15 a return circuit arises, and becomes a factor of the radiation noise. Here, the signal ground 15 shown in FIG. 6 means, for example, a GND plane formed on a printed wiring board, or the like.
There is a method using a common mode choke 10 as means of coping with the radiation noise resulting from this common mode current, as shown in FIG. 7.
Since the common mode choke 10 has the structure that the two signal wiring 4a and 4b are wound in the same direction around the same core, magnetic fields cancels each other in regard to signal component currents which flow in reverse directions mutually, and those signal component currents are passed. Nevertheless, since magnetic fields are added in regard to the common mode noise components, which flow in the same direction, to increase each other, it has impedance property, and hence, it has property of making the common mode current hard to flow through.
However, when the common mode choke 10 is used to be inserted at some points in the signal wiring 4a and 4b, characteristic impedances of the signal wiring 4a and 4b differ at the insertion points. Then, signals transmitted on the signal wiring 4a and 4b are reflected by the common mode choke 10, and transmitted signal waveforms are distorted. A common mode choke coil which suppresses the reflection at the insertion points of this common mode choke 10 to prevent the distortion of the transmitted signal waveforms is also proposed (for example, refer to Japanese Patent Laid-Open No. 2000-58343).
FIG. 8(a) is a perspective view showing appearance of the common mode choke coil disclosed in Japanese Patent Laid-Open No. 2000-58343, and FIG. 8(b) shows an exploded perspective view of the common mode choke coil, respectively.
This common mode choke coil is constituted of coil conductors 22a and 22b being bifilar-wound a ring-shaped toroidal core 20 made of ferrite, and this toroidal core 20 being contained in an armor case constituted of a lid unit 16 and a case unit 21.
The case unit 21 is constituted of a cylindrical inner circumferential wall 21a and an outer circumferential wall 21b being joined with a bottom wall 21c, and has a ring-shaped containing unit 21d for containing the toroidal core 20 inside. The lid unit 16 is disc-shaped and blocks up the containing unit 21d of the case unit 21. And, four claws 16b are drawn out at equal intervals from a marginal portion of the lid unit 16 along an outer side face of the outer circumferential wall 21b of the case unit 21.
A ground conductor 17 which is constituted of a chromium plating membrane and the like is formed on an outer side face of the outer circumferential wall 21b and the outside face of the bottom wall 21c of the case unit 21, and the outside face of the lid unit 16. On the ground conductor 17 of the outer circumferential wall 21b and the bottom wall 21c of the case unit 21, four locations of insulating layers 18 which are made of a resin which has insulation property, or the like are formed at equal intervals. On the insulating layers 18, terminal plates 19 which are made of a metal material such as phosphor bronze are mounted, respectively. End portions of the coil conductors 22a and 22b are soldered to these four terminal plates 19, respectively.
The lid unit 16 is fixed to the case unit 21 by making their claws 16b catch the bottom wall 21c of the case unit 21 together.
The common mode choke coil constituted in this way is used as the common mode choke 10 in FIG. 7 by the signal wiring 4a and 4b being connected to the terminal plates 19 respectively. Then, since the grounded ground conductor 17 faces the coil conductors 22a and 22b with sandwiching the armor case which is constituted of the lid unit 16 and case unit 21, electrostatic capacity (distributed capacity) which uses the armor case as a dielectric is formed among these. An LC distributed constant circuit is formed between each of the coil conductors 22a and 22b and a ground by this electrostatic capacity and inductor which the coil conductors 22a and 22b have. This electrostatic capacity is determined by a dielectric constant of the resin which constitutes the armor case, and a facing area and a distance of the ground conductor 17 and coil conductors 22a and 22b. Hence, it is possible to make characteristic impedance between each of the coil conductors 22a and 22b and the ground coincide with characteristic impedance between each of the signal wiring 4a and 4b and the ground by selecting adequately these values. Thus, it is possible to suppress reflection of the signal in the common mode choke 10.
Nevertheless, in high-speed transmission in recent years, even if the common mode choke 10 is used like the conventional, a sufficient noise reduction effect is no longer obtained.
As a factor, it has turned out that not only the common mode currents flowing with making the two signal wiring 4a and 4b outward circuits, and making the signal ground 15 a return circuit, but secondary common mode currents flowing in the same direction on the two signal wiring 4a and 4b and signal ground 15 are a noise factor. In regard to the noise resulting from this secondary common mode current, the conventional common mode choke 10 cannot produce a sufficient reduction effect.
Secondary common mode currents will be explained using FIG. 9. FIG. 9(a) is an explanatory diagram about common mode currents, and FIG. 9(b) is an explanatory diagram about the secondary common mode currents. Here, the case that the two signal wiring 4a and 4b which form a differential transmission line are formed on a printed wiring board will be explained as an example. In addition, the same reference numerals are used for the same components as those in FIG. 6.
FIG. 9(a) is a drawing showing a flow of the differential mode current and common mode currents in differential transmission. What flow in reverse directions between the signal wiring 4a and 4b are differential mode currents, and when respective signal wiring 4a and 4b are in complete equilibrium (distances from the signal ground 15 are the same, the width of the two signal wiring 4a and 4b are equal, and the like) to the signal ground 15, common mode currents are not generated.
However, when some unbalance (difference in width, difference in length, or the like) exists between the two signal wiring 4a and 4b, the common mode currents which flow in the same direction between the two signal wiring 4a and 4b as shown in FIG. 9(a) are generated.
FIG. 9(b) is a drawing showing generation of the secondary common mode currents resulting from discontinuity of the signal ground 15. A reference ground 23 provides a reference potential of the signal ground 15, and it is assumed that an enclosure provides a reference potential of the ground (signal ground 15) of the printed wiring board here.
In consideration of what is similar to the two signal wiring 4a and 4b in differential transmission in FIG. 9(a), currents flowing with making the two signal wiring 4a and 4b outward circuits, and making the signal ground 15 a return circuit are common mode currents. Then, when unbalance (width of the signal ground 15 is discontinuous, or the like) exists in them, the secondary common mode currents which flows through the two signal wiring 4a and 4b and signal ground 15 in the same direction as shown in FIG. 9(b) flow as a next mode.
The present invention aims at providing the radiation noise suppression circuit of the differential transmission line which solves the conventional subject mentioned above, does not deteriorate the reduction effect of conventional primary common mode current, and can reduce secondary common mode currents.